The present invention relates to a stencil printing method including the steps of applying ink on one surface of a stencil sheet having perforations therein, disposing a sheet or material to be printed on the other surface of the stencil sheet, and transferring ink onto the sheet by forcing ink to pass through the perforations; and, further to a stencil printing machine for use in the method.
In case where a desired image is formed by using a stencil sheet, a conventional printing method is adopted, which includes the steps of applying ink such as emulsion ink on one surface of a stencil sheet with perforations, disposing a printing sheet on the other surface of the stencil sheet, and transferring the ink onto the printing sheet by exerting pressure on the one surface to force the ink to pass through the perforations, thereby completing printing.
In the printing operation described above, the pressure for transferring ink is applied on the surface by using methods such as a flat press method and a squeeze method. In the flat press method, ink is applied on a surface of a stencil sheet, and then the surface is pressed in a flat condition, so that the ink is extruded through the perforated images of the stencil sheet. In the squeeze method, ink is applied on a surface of a stencil sheet, and then pressed by moving a squeezing plate along the surface, thereby to allow the ink to pass through the perforated images of the stencil sheet. Microscopic observation of these methods shows that the pressurized ink portions press another ink portions disposed below. That is, the ink itself functions as a pressure transmitting substance for extruding the ink.
By the way, as to the ink used in the methods described above, as the fluidity or softness of ink is increased, it permeates further into the printing sheet. This decreases a drawback caused by set-off. In this situation, however, capillarity phenomenon arises between the stencil sheet and the printing sheet, thereby causing an excessive-ink flow and ink bleeding to deteriorate quality of the printed images.
Conversely, ink with low fluidity and softness does not cause capillarity phenomenon easily, thereby improving quality of the printed images. In this situation, however, there arises another problem such that it takes a long time to permit the ink to permeate into the printing sheet after ink-transfer on the printing sheet. Further, as illustrated in FIG. 10 (a), when a printing sheet 32 is separated from a stencil sheet 31 upon completion of printing, the two sheets are pulled back to each other by ink 33 with a high viscosity. As an area of a perforated image 31a is increased, the phenomenon of the pulling-back by the ink appears more. Consequently, as illustrated in FIG. 10 (b), excessive ink is transferred to the printing sheet 32, thereby causing a set-off phenomenon where a back of the sheet is stained when it is stacked after printing.
Accordingly, an object of the present invention is to provide a stencil printing method and a stencil printing machine which overcome the contradictory phenomena previously explained, and can provide stencil printing with an excellent printing quality and less set-off by avoiding excessive ink transfer on a printed sheet or material.